1. The contribution of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) to non-adrenergic non-cholinergic (NANC) relaxations in the pig gastric fundus was investigated. 2. Circular and longitudinal muscle strips were mounted for isotonic registration in the presence of atropine and guanethidine; tone was raised with 5-hydroxytryptamine. Electrical field stimulation with 10 s trains at 5 min intervals induced responses were abolished by tetrodotoxin. 3. The short-lasting as well as the sustained electrically induced NANC relaxations were significantly reduced by NG-nitro-L-arginine methyl ester (L-NAME). Pretreatment with L-arginine but not D-arginine, prevented the inhibitory effect of L-NAME except for sustained relaxations in the longitudinal muscle strips. 4. Sodium nitroprusside, forskolin, zaprinast and 3-isobutyl-l-methylxanthine induced concentration-dependent relaxations. Exogenous NO mimicked the short-lasting electrically induced relaxations, while endogenous VIP evoked sustained relaxations. The responses to exogenous NO and VIP were not influenced by tetrodotoxin and L-NAME. 5. alpha-Chymotrypsin abolished the responses to exogenous VIP but only moderately reduced NANC relaxations induced by continuous electrical stimulation. Zaprinast potentiated the relaxant responses to sodium nitroprusside and increased the duration of the NANC relaxations induced by electrical stimulation with 10 s trains in circular muscle strips but not in longitudinal muscle strips. 6. The cyclic GMP and cyclic AMP response to electrical stimulation, NO and VIP was measured in circular muscle strips. Short-lasting as well as sustained electrical field stimulation induced an approximately 1.5 fold increase in cyclic GMP content, while NO induced nearly a 40 fold increase. An increase in cyclic AMP content was obtained only with sustained electrical field stimulation. 7. Immunocytochemistry for NO synthase (NOS) revealed immunoreactive neuronal cell bodies in the submucous and myenteric plexuses and nerve fibres in both the circular and longitudinal muscle layer; double-labelling for NOS and VIP showed that VIP coexists in a major part of the intrinsic neurones. NADPH diaphorase-histochemistry showed the same pattern of nitrergic neurones and nerves as NOS-immunocytochemistry. 8. It is concluded that a cyclic GMP- and a cyclic AMP-dependent pathway for relaxation is present in both the circular and longitudinal muscle layer of the pig gastric fundus. NO appears to contribute to short-lasting as well as sustained NANC relaxations. A peptide, possibly VIP, may be involved during sustained stimulation at lower frequencies of stimulation.
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